Liquid crystal display device

ABSTRACT

According to one embodiment, a lateral electric-field type of liquid crystal display device includes a display panel and a controller, wherein a frame frequency falls within a range of 1 Hz to 10 Hz, an off-leak current of each of the TFTs has a value of 1×10 −15  A or less, a resistivity of a liquid crystal and a resistivity of an alignment film both fall within one of a first range and a second range, the first range being 1×10 13  to 5×10 13  Ω·cm, the second range being 5×10 13  to 5×10 14  Ω·cm, and a relationship of “R1×C1≈R2×C2” is satisfied, where R 1  is a resistance and C 1  is a capacity with respect to each pixel, R 2  is a resistance and C 2  is a capacity with respect to each pixel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2013-213566, filed Oct. 11, 2013, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a liquid crystaldisplay device.

BACKGROUND

Liquid crystal display devices are provided in various devices, forexample, a television device, a vehicle-mounted display such as a carnavigation device, and mobile terminals such as a notebook personalcomputer, a tablet PC, a cell phone, and a smart phone.

Such liquid crystal display devices adopt various mode liquid crystalswhich are applied in accordance with purposes.

For example, in a vertical electric field type of liquid crystal displaydevice such as a twisted nematic (TN) mode liquid crystal display deviceor an optically compensated bend (OCB) mode liquid crystal displaydevice, an alignment direction of liquid crystal molecules included in aliquid crystal layer held between an upper substrate and a lowersubstrate is controlled by an electric field generated between acounter-electrode provided in the upper substrate and pixel electrodesprovided in the lower substrate.

Furthermore, in a lateral electric field type of liquid crystal displaydevice such as an in-plane switching (IPS) mode liquid crystal displaydevice or a fringe-field switching (FFS) mode liquid crystal displaydevice, a counter-electrode (referred to as a COM electrode in thistype) and pixel electrodes are provided in the same substrate, and analignment direction of liquid crystal display molecules included in aliquid crystal layer is controlled by an electric field (fringe electricfield) generated between the counter-electrode and the pixel electrodes.The FFS mode liquid crystal display device can ensure a great apertureratio, and thus has a high luminance and a good viewing anglecharacteristic.

It should be noted that a liquid crystal display device for use in amobile terminal is strongly required to reduce the power consumption ofa circuit, and a low-frequency driving method, an intermittent drivingmethod, etc. are proposed as means for reducing the power consumption ofthe circuit. The low-frequency driving method is a method of reducingthe power of the circuit by lowering a drive frequency of the liquidcrystal display device to, e.g., ½ or ¼ of that under standardconditions. The intermittent driving method is a method of reducing thepower of the circuit by stopping the circuit for a time periodcorresponding to several display time periods after writing is performedfor a single display time period. In both those driving methods, aperiod of rewriting a video signal in a liquid crystal display unit islong. Thus, those driving methods are not suitable for displaying ofmoving images. However, they can be effectively applied, as methods forlowering the power of the circuit, to displaying of a still image or thelike whose visibility is held in little account.

If the low-frequency driving method or the intermittent driving methodis applied to a liquid crystal display device, it is necessary to reducea flicker.

For example, in the case where a frame frequency was 60 Hz, which isapplied in an ordinary liquid crystal display device, a flicker was notvisibly recognized. On the other hand, in the case where the framefrequency was 20 Hz, which is ⅓ of 60 Hz, a flicker was visiblyrecognized. Furthermore, in the case where the frame frequency wasfurther lowered, a flicker was more remarkably visibly recognized.

BRIEF DESCRIPTION OF THE DRAWINGS

A general architecture that implements the various features of theembodiments will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrate theembodiments and not to limit the scope of the invention.

FIG. 1 is a plan view schematically showing a structure of a liquidcrystal display device according to an embodiment;

FIG. 2 is a view showing a cross section of a display pixel portion of aliquid crystal display panel of the liquid crystal display deviceaccording to the embodiment;

FIG. 3A is a view showing an example of a luminance change with respectto the liquid crystal display device according to the embodiment;

FIG. 3B is another view showing the example of the luminance change withrespect to the liquid crystal display device according to theembodiment;

FIG. 4 is a view obtained by subjectively evaluating a relationshipbetween a frame frequency and an upper limit of a standardizedfluctuation range PP value of a luminance in which a flicker is notvisibly recognized;

FIG. 5 is a view showing fluctuation ranges of luminances respect toTFTs whose leak currents are different from each other, the view of FIG.4 showing characteristics being applied to FIG. 5.

FIG. 6A is a view showing a luminance response waveform in the liquidcrystal display according to the embodiment;

FIG. 6B is a view for use in explaining a symmetric component withrespect to the liquid crystal display device according to theembodiment;

FIG. 6C is a view for use in explaining an antisymmetric component withrespect to the liquid crystal display device according to theembodiment;

FIG. 7A is a view showing an equivalent circuit in the liquid crystaldisplay panel of the liquid crystal display device according to theembodiment;

FIG. 7B is a view for use in explaining a variation of a liquid crystalholding voltage, which is caused by an impedance mismatch between liquidcrystal and an alignment film in the liquid crystal display deviceaccording to the embodiment;

FIG. 8A is a view showing a circuit model applied to a simulation withrespect to the liquid crystal display device according to theembodiment;

FIG. 8B is a view showing calculation conditions applied to thesimulation with respect to the liquid crystal display device accordingto the embodiment;

FIG. 9A is a view showing a result of the simulation with respect to achange rate of a liquid crystal applying voltage in a holding period inthe liquid crystal display device according to the embodiment;

FIG. 9B is a view showing the change rate of the liquid crystal displaydevice according to the embodiment;

FIG. 10 is a view showing for reducing a flicker in the liquid crystaldisplay device according to the embodiment;

FIG. 11 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 1 and 2 in the liquid crystaldisplay device according to the embodiment;

FIG. 12 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 3 and 4 in the liquid crystaldisplay device according to the embodiment;

FIG. 13 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 5 and 6 in the liquid crystaldisplay device according to the embodiment;

FIG. 14 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 7 and 8 in the liquid crystaldisplay device according to the embodiment;

FIG. 15 is a view showing regions regarding impedances of a liquidcrystal and an alignment film in case 9 in the liquid crystal displaydevice according to the embodiment; and

FIG. 16 is a view showing regions regarding impedances of a liquidcrystal and an alignment film in case 10 in the liquid crystal displaydevice according to the embodiment.

DETAILED DESCRIPTION

Various embodiments will be described hereinafter with reference to theaccompanying drawings.

In general, according to one embodiment, a lateral electric-field typeof liquid crystal display device comprising: a display panel in which aplurality of liquid crystal pixels and TFTs are arranged in a matrix,the TFTs being configured to perform or stop application of pixelvoltages to the liquid crystal pixels; and a controller configured toperform intermittent driving to rewrite image signals to the liquidcrystal pixels, wherein a frame frequency falls within a range of 1 Hzto 10 Hz, an off-leak current of each of the TFTs has a value of 1×10⁻¹⁵A or less, a resistivity of a liquid crystal applied to each of theliquid crystal pixels and a resistivity of an alignment film applied tothe liquid crystal pixels both fall within one of a first range and asecond range, the first range being set as a range of 1×10¹³ to 5×10¹³Ω·cm, the second range being set as a range of 5×10¹³ to 5×10¹⁴ Ω·cm,and a relationship of “R1×C1≈R2×C2” is satisfied, where R1 is aresistance of the liquid crystal with respect to said each liquidcrystal pixel, C1 is a capacity of the liquid crystal with respect tosaid each liquid crystal pixel, R2 is a resistance of the alignment filmwith respect to said each liquid crystal pixel, and C2 is a capacity ofthe alignment film with respect to said each liquid crystal pixel.

The liquid crystal display device according to the embodiment will beexplained in detail with reference to the accompanying drawings.

FIG. 1 is a schematic plan view showing a structure of the liquidcrystal display device according to the embodiment.

The liquid crystal display device comprises a liquid crystal displaypanel PNL and a backlight BLT which illuminates the liquid crystaldisplay panel PNL from a rear surface side. The liquid crystal displaypanel PNL is provided with a display portion including display pixels PXarranged in a matrix.

FIG. 2 is a view showing a cross section of a display pixel portion ofthe liquid crystal display panel PNL of the liquid crystal displaydevice according to the embodiment.

The liquid crystal display panel PNL comprises an array substrate 100, acounter-substrate 200 and a liquid crystal layer LQ held between thesubstrates 100 and 200.

In the counter-substrate 200, a transparent insulating substrate SB2, acolor filter layer CF and an overcoat layer L2 are provided. The colorfilter layer CF includes colored layers provided on the transparentinsulating substrate SB2, which are colored red (R), green (G) and blue(B). The overcoat layer L2 is provided to cover the color filter layerCF, and prevents substances contained in the color filter layer CF fromflowing into the liquid crystal layer LQ.

The array substrate 100 comprises a transparent insulating substrateSB1, a counter-electrode (first electrode) COM, and a plurality of pixelelectrodes (second electrodes) PE. The pixel electrodes PE are providedon the counter-electrode COM, with an insulating layer L1 interposedbetween the pixel electrodes PE and the counter-electrode COM, theinsulating layer L1 being formed of silicon nitride (SiN). The pixelelectrodes PE are provided in display pixels PX, respectively, andinclude opening portions SLT each formed in the shape of a slit. Thecounter-electrode COM and the pixel electrodes PE are transparentelectrodes formed of, e.g., indium tin oxide (ITO).

As shown in FIG. 1, at the display portion, the array substrate 100includes scanning lines GL (GL1, GL2, . . . ) extending along columns ofdisplay pixels PX, signal lines (SL1, SL2, . . . ) extending along rowsof display pixels PX, and pixel switches SW provided close tointersections of the scanning lines GL and the signal lines SL.

The pixel switches SW comprise thin film transistors (TFTs). The pixelswitches SW include gate electrodes which are electrically connected toassociated scanning lines GL, respectively. Also, the pixel switches SWinclude source electrodes which are electrically connected to associatedsignal lines SL, respectively. Furthermore, the pixel switches SWinclude drain electrodes which are electrically connected to associatedpixel electrodes PE, respectively.

The array substrate 100 comprises a source driver SD and gate drivers GD(a left gate driver GD-L and a right gate driver GD-R) as drive meansfor driving the display pixels PX. The scanning lines GL areelectrically connected to output terminals of the gate drivers GD. Thesignal lines SL are electrically connected to output terminals of thesource driver SD.

The gate drivers GD and the source driver SD are provided in peripheralareas of the display portions. The gate drivers GD successively applyon-voltages to scanning lines GL, as a result of which the on-voltagesare applied to the gate electrodes of the pixel switches SW, which areelectrically connected to selected scanning lines GL, i.e., the abovescanning lines GL. To be more specific, when an on-voltage is applied toa gate electrode, electrical conduction is effected between the sourceelectrode and drain electrode of a pixel switch SW including the abovegate electrode. On the other hand, the source driver SD supplies outputsignals to the signal lines SL, respectively. To be more specific, whenan output signal is supplied to a signal line SL, it is also supplied,through the pixel switch SW in which electrical conduction is effectedbetween its source and drain electrodes, to an associated pixelelectrode PE.

Operations of the gate drivers GD and the source driver SD arecontrolled by a control circuit CTR provided outside the liquid crystaldisplay panel PNL. Furthermore, the control circuit CTR applies acounter-voltage Vcom to the counter-electrode COM, and also controls anoperation of the backlight BLT.

The control circuit CTR has a function of performing intermittentdriving to reduce electric power, in addition to a function ofperforming ordinary driving. It should be noted that a time period inwhich a single frame is rewritten is referred to as a “frame period” andits reciprocal is as a “frame frequency”. This will also be applied tothe intermittent driving and low-frequency driving of the presentapplication.

Suppose by way of example a standard frame frequency of the liquidcrystal display device is 60 Hz (a single frame is rewritten every 1/60sec). In the case of displaying moving images, the liquid crystaldisplay device is operated at 60 Hz, which is the standard framefrequency, and in the case of displaying a still image or the like whosevisibility is held in little account, the control circuit CTR performsintermittent driving.

The control circuit CTR sets a non-operation period of, e.g., 1/60 sec,3/60 sec, 7/60 sec or 59/60 sec after performing a writing operation(scanning from an upper side of a screen to a lower side thereof) for1/60 sec. In the non-operation period, by stopping the writing operationof the control circuit CTR, the power consumption of the circuit is madesubstantially zero. As a result, the time-averaged power consumption ofthe circuit in all time periods including the time of performing thewriting operation is reduced to 1/2, 1/4, 1/8 or 1/60 of that in thecase where the non-operation period is not provided.

The liquid crystal display device according to the embodiment is afringe-field switching (FFS) mode liquid crystal display device in whicha liquid crystal layer LQ is caused to generate an electric field due toa potential difference between voltages applied to the counter-electrodeCOM and the pixel electrodes PE, to control an alignment direction ofliquid crystal molecules in the liquid crystal layer. The transmissionamount of light emitted from the backlight BLT is controlled inaccordance with the alignment direction of the liquid crystal molecules.

As shown in FIG. 2, a capacitance component CsO generates between eachof the pixel electrodes PE and the counter-electrode COM, which arelocated opposite to each other, with the insulating layer L1 interposedbetween them. In addition, a liquid crystal capacitance Clc and anauxiliary capacitance component Cs1 corresponding to an electric fieldgenerated in the liquid crystal layer LQ are present. Where a totalcapacitance, which corresponds to all the capacitances between the pixelelectrodes PE and the counter-electrode COM, is denoted by Cs, thecapacitance Cs is considered to be provided between the drain of the TFTand the counter-electrode COM.

Next, a driving method for reducing a flicker will be explained.

If a DC voltage is applied to liquid crystal material for long time, adisplay characteristic thereof varies due to charging-up with thepassage of time. Thus, in general, frame driving is applied. In theframe driving, driving is performed such that the polarity is invertedin units of one frame to make a DC average substantially zero. However,if positive and negative response characteristics (luminance-voltagecharacteristics) are different from each other, the luminances ofpositive and negative frames are also different from each other, and theluminance fluctuates each time the frame changes, thus causing aflicker. Although the flicker can be minimized by applying a minuteoffset voltage for an average (DC average value) of signals inverted inpolarity or adjusting a potential of the counter-electrode, it isimpossible to suppress the flicker by completely absorbing a change ofthe brightness-voltage characteristic which occurs as time passes, avariance in optimal condition between gradation levels, etc.

As measures for reducing such a flicker, e.g., a line-inversion drivescheme, a column-inversion drive scheme and a dot-inversion drive schemeare known. For example, in the line-inversion drive scheme, a temporalphase of polarity inversion is set such that the polarity is invertedbetween positive and negative polarities in units of one row, therebycausing the difference between positive and negative luminance responsesto be macroscopically zero, and preventing the flicker from beingvisibly recognized. Similarly, in the column-inversion drive scheme, thephase of polarity inversion is reversed in units of one column, and inthe dot-inversion drive scheme, the pixel electrodes PE arranged in amatrix are inverted in polarity in a checkerwise manner; that is, dotinversion is carried out. Thus, the column-inversion drive scheme andthe dot-inversion drive scheme can also prevent a flicker from beingvisibly recognized.

Of those inversion drive schemes, the line-inversion drive scheme andthe dot-inversion drive scheme are carried out such that at the time ofperforming scanning over the screen, writing to pixels is carried outwhile inverting the polarity in units of one row, as a result of whichit is therefore necessary to perform charging and discharging of signallines in a panel at intervals of 1 horizontal period (1 H period), andthe power consumption is increased. On the other hand, in thecolumn-inversion drive scheme, the polarity is not inverted in a columndirection where rows of pixels are arranged, and the power consumptionof the circuit is reduced. In this regard, the column-inversion drivescheme has an advantage. In a liquid crystal display device for use in amobile device, although any of various inversion drive scheme is adoptedin accordance with specifications of a product, it is most preferablethat the column-inversion drive scheme be applied in order to reduce thepower consumption.

Then, a luminance response waveform of the liquid crystal display devicewill be explained.

In the intermittent drive scheme, after an associated pixel voltage isapplied to a respective one of pixels PX, it is necessary that the pixelvoltage is held for a long time period. Thus, it is also necessary totake measures for a luminance change which occurs due to an off-leakcurrent of a TFT.

FIGS. 3A and 3B are views showing an example of a luminance change inthe liquid crystal display device according to the embodiment. To bemore specific, FIG. 3A shows changes of a pixel voltage Vd, a signalline voltage Vs and a gate voltage Vg; and FIG. 3B shows a change of aluminance of a liquid crystal.

In a scanning period of a single frame, the source driver SD outputssignal line voltages Vs associated with image signals to signal linesSL. Then, the gate driver GD outputs a gate voltage Vg for turning onTFTs to a scanning line GL. Thereby, pixel voltages Vd are equalized tothe signal line voltages Vs, as a result of which the luminances ofpixels are each set to a desired value.

In a non-operation period of the single frame, the gate voltage Vg is ata level at which it turns off TFTs, and for a short time period, thepixel voltages Vd are held at the levels of the signal line voltages Vs.However, if the non-operation period is long, the pixel voltages Vdlower due to the off-leak currents of the TFTs (currents leaking andflowing from, for example, drains of the TFTs to sources thereof). As aresult, in the non-operation period, the luminances of the pixels lower.

When a subsequent frame is started, and the pixel voltages Vd arerewritten with new signal line voltages Vs, the luminances of the pixelseach return to the desired value. It should be noted that referring toFIG. 3A, the polarity of a pixel voltage Vd changes in a subsequentframe. This is because a frame-inversion driving is performed. Aluminance change, which is caused by off-leak currents of TFTs, isrepeatedly made each time the frame changes. Thus, a flicker is visiblyrecognized.

It should be noted that a luminance as shown in FIG. 3B is standardizedsuch that the desired value is 1. The value by which the standardizedluminance is lowered by the off-leak current will be referred to as astandardized fluctuation range PP value of the luminance.

It is known that in general, the visibility with which a person can viewa flicker depends on a frequency, and with respect to frequencies, evenif the luminance fluctuates in the same fluctuation range, a flicker ismore easily visibly as the frequency lowers.

FIG. 4 is a view obtained by subjectively evaluating a relationshipbetween a frame frequency and an upper limit of a standardizedfluctuation range PP value of the luminance in which a flicker cannot bevisibly recognized. According to a graph shown in the view, the upperlimit steeply rises when the frame frequency exceeds 40 Hz. This meansthat in the case where the frame frequency is greater than 40 Hz, evenif the fluctuation range of the luminance is great, a flicker is notvisibly recognized; however, in the case where the frame frequency isequal to or smaller than 40 Hz, even if the fluctuation range of theluminance is small, a flicker is visibly recognized.

From the above result of the subjective evaluation, it can be consideredthat if the luminance fluctuates to such a degree as shown in FIG. 3B, aflicker is not visibly recognized when the frame frequency is 60 Hz,which is an ordinary frame frequency; however, a flicker is visiblyrecognized when the frame frequency is lowered to 40 Hz or less.

FIG. 5 is a view showing a luminance fluctuation occurring in TFTs whoseleak currents are different in value from each other, with the view ofFIG. 4 applied to the view of FIG. 5. Curve A indicates a relationshipbetween the frame frequency and the fluctuation range of the luminancein the case where a leak current (I (off)) is 1×10⁻¹⁵ (A). Curve Bindicates also a relationship between the frame frequency and thefluctuation range of the luminance in the case where the leak current (I(off)) is 1×10⁻¹³ (A) which is a characteristic of a conventional TFT.Curve C indicates a relationship between the frame frequency and thefluctuation range of the luminance in the case where the leak current (I(off)) is 1×10⁻¹² (A).

With respect to curves A-C, a flicker is visibly recognized in a regionlocated upper than a character curve L indicating an upper limit of aregion at which a flicker is not visibly recognized. It can be seen thatas the leak current increases, at a higher frame frequency, a flicker isvisibly recognized. Furthermore, in curve B (the conventional TFT), at aframe frequency of 40 Hz or less, a flicker is visibly recognized. Inorder that a flicker should not be visibly recognized at 1 Hz, it isnecessary as a requirement that as indicated in curve A, the leakcurrent is 1×10⁻¹⁵ (A) or less.

Therefore, it is possible to prevent a flicker from being visiblyrecognized, by selecting a TFT to be applied, in accordance with a framefrequency band to be applied, based on FIG. 5.

A symmetric component and an antisymmetric component to be applied instudying how to handle a luminance fluctuation at the time of performinga low-frequency driving will be explained.

FIGS. 6A-6C are views for use in explaining a symmetric component and anantisymmetric component in the liquid crystal display device accordingto the embodiment.

FIG. 6A shows a luminance response of a single pixel in the case wheredriving is performed in a frame period of 50 msec (at a frame frequencyof 20 Hz). In the figure, a vertical axis indicates luminance, and ahorizontal axis indicates time. It should be noted that the luminance isstandardized such that an average value is 1. The polarity of a videosignal to be written to the pixel is inverted in units of one frame, andintervals indicated by arrows in the figure correspond to a negativeframe and a positive frame, respectively.

In the luminance response as shown in FIG. 6A, in the positive frame,the luminance is increased to approximately 1.3, and in the negativeframe, the luminance is decreased to approximately 0.8. Thus, theluminance in the positive frame is greatly different from that in thenegative frame. This is because the response of a liquid crystal variesin accordance with whether a voltage having a positive polarity or thathaving a negative polarity is applied.

It should be noted that the symmetric component corresponds to anaverage waveform of the luminance waveform of the positive frame andthat of the negative frame. In the symmetric component, each time theframe changes, the same waveform (symmetric waveform) is obtained. Thatis why it is referred to as a symmetric component. FIG. 6B shows anexample of the symmetric component. If the symmetric waveform has a flatcharacteristic, it means that it is averaged to have a flatcharacteristic. Thus, when driving is performed in the line-inversiondrive scheme, the column-inversion drive scheme or the dot-inversiondrive scheme in the above manner, it can be visibly recognized that thefluctuation range of the luminance is small (a flicker does not occur).However, the symmetric component does not have a flat characteristic.Therefore, the symmetric component is a component which is noteliminated even by the inversion drive scheme.

On the other hand, the antisymmetric component corresponds to a waveformwhich is expressed, after determining an average waveform of theluminance waveform of the positive waveform and that of the negativewaveform, with the average waveform determined as zero (a reference).Therefore, a positive waveform and a negative waveform appear. That iswhy the above component is referred to as an antisymmetric component.FIG. 6C shows an example of the antisymmetric component. If theantisymmetric component has a symmetric characteristic with respect to areference line, it is visibly recognized as a component in which theluminance does not fluctuate (no flicker occurs), when driving isperformed in the line-inversion drive scheme, the column-inversion drivescheme or the dot-inversion drive scheme. The antisymmetric component issymmetric with respect to the reference line. Therefore, theantisymmetric component is a component which can be eliminated by theinversion drive scheme.

According to the above study, the luminance fluctuation due to the leakcurrent of the TFT is the luminance fluctuation of the symmetriccomponent. Thus, this cannot be prevented by the inversion driving.

The above explanation is given with respect to a current leak of a TFTdue to which a flicker is visibly recognized at the time of performingthe low frequency driving. However, a flicker caused by a cell is alsovisibly recognized; that is, it is another cause for a flicker visiblyrecognized. The inventors made researches on how to restrict a flickeroccurring due to a cell, and clarified that occurrence of a flicker isinfluenced by impedances of a liquid crystal and an alignment film.

FIGS. 7A and 7B are views for use in explaining a variation of a liquidcrystal holding voltage which occurs due to an impedance mismatchbetween a liquid crystal and an alignment film with respect to theliquid crystal display device according to the embodiment.

A model of an equivalent circuit of a liquid crystal panel, which isshown in FIG. 7A, includes a holding capacitance Cs which is present inan region with respect to which a pixel electrode PE and acounter-electrode COM are located opposite to each other, a capacitanceC2 of an alignment film which corresponds to an electric field generatedin the liquid crystal layer LQ, and a liquid crystal capacitance C1.Furthermore, resistances R1 and R2 are provided in parallel with theliquid crystal capacitance C1 and the capacitance C2, respectively. Itshould be noted that the holding capacitance Cs, the capacitance C2 andthe liquid crystal capacitance C1 correspond to the capacitancecomponent Cs0, Cs1 and Clc as shown in FIG. 2.

Next, a variation of a liquid crystal holding voltage which occurs dueto the impedance mismatch between the liquid crystal and the alignmentfilm will be explained with reference to FIG. 7B.

In case 1, impedances of a liquid crystal and an alignment film do notmatch to each other (R1·C1<R2·C2). In this case, when a TFT is turnedoff to be in a pixel voltage holding state, the liquid crystal and thealignment film are discharged independently, but charge is supplied fromthe holding capacitance Cs in such a manner as to compensate forattenuation of voltages V1 and V2 which is caused by the abovedischarge. However, although the amount of charge decreased due to thedischarge on the liquid crystal holding voltage V1 side is larger thanthat on the voltage V2 side, the amount of charge supplied on the V1side is equivalent to that on the V2 side. Therefore, with the passageof time, the voltage V1 is decreased, and the voltage V2 is increased.Therefore, in case 1, the liquid crystal holding voltage attenuates, andthe luminance fluctuates.

In case 2, impedances of a liquid crystal and an alignment film match toeach other (R1·C1≈R2·C2). In this case, when a TFT is turned off to bein a pixel voltage holding state, the liquid crystal and the alignmentfilm are discharged independently. However, discharging time of theliquid crystal side and that of the alignment film side are the same aseach other, since the impedances match to each other. Thus, decreasingof charge due to the discharge and supplying of charge from the holdingcapacitance Cs are balanced, thus keeping the voltages V1 and V2constant. Therefore, in case 3, the fluctuation of the luminance isreduced, and the flicker is reduced.

In case 3, impedances of a liquid crystal and an alignment film do notmatch to each other (R1·C1>R2·C2). In this case, when a TFT is turnedoff to be in a pixel voltage holding state, the liquid crystal and thealignment film are discharged independently, but charge is supplied fromthe holding capacitance Cs in such a manner as to compensate forattenuation of voltages V1 an V2 which is caused by the above discharge.However, although the amount of charge decreased due to the discharge onthe alignment film voltage V2 side is larger than that on the voltage V1side, the amount of charge supplied on the V2 side is equivalent to thaton the V1 side. Therefore, with the passage of time, the voltage V2 isdecreased, and the voltage V1 is increased. Therefore, in case 3, theliquid crystal holding voltage increases, and the luminance fluctuates.

The variation of the liquid crystal holding voltage is confirmed bymaking a simulation calculation with respect to the above circuit model.

FIGS. 8A and 8B are views showing a circuit model applied to asimulation performed with respect to the liquid crystal display deviceaccording to the embodiment and calculation conditions. FIG. 8A showsthe circuit model, and FIG. 8B shows calculation conditions. It shouldbe noted that with respect to the calculation conditions, a frame periodof 0.5 seconds (frame frequency of 2 Hz) is applied to intermittentdriving.

FIGS. 9A and 9B are views showing a result of a simulation performedwith respect to a change rate of a liquid crystal applied voltage in aholding period in the liquid crystal display device according to theembodiment. FIG. 9A shows a change rate of the liquid crystal appliedvoltage in association with a resistivity of the alignment film and thatof the liquid crystal. It should be noted that the change rate of theliquid crystal applied voltage is an index indicating a rate of adecreased or increased liquid crystal applied voltage in the holdingperiod, and is a value as (Vf−Vi)/Vi as shown in FIG. 9B.

A center portion in FIG. 9A is a region in which the change rate of theliquid crystal applied voltage is the smallest. The change rate of theliquid crystal applied voltage increases as the resistivity of thealignment film decreases, as indicated by an arrow of a solid line inthe figure. On the other hand, the change rate of the liquid crystalapplied voltage decreases as the resistivity of the liquid crystaldecreases, as indicated by an arrow of a dotted line in the figure.Also, from this result of the simulation, it can be seen that aluminance fluctuation in a single frame can be reduced by matching theimpedances of the liquid crystal and the alignment film to each other.

It should be noted that an upper limit of an absolute value of aluminance gradient in a single frame in which a flicker is not visiblyrecognized is determined as 0.03 by a subjective evaluation. Therefore,it is possible to set the absolute value of the luminance gradient inthe single frame to 0.03 or less by selecting resistivities (Ω·cm) ofthe liquid crystal and the alignment film based on the simulation resultas shown in FIG. 9A.

For the reason given in the above explanation, it is possible to achievea liquid crystal display device in which a flicker is reduced bycombining an appropriate frame frequency, appropriate TFT off-leakcurrent and appropriate impedances of a liquid crystal and an alignmentfilm.

FIG. 10 is a view showing a structure for reducing a flicker in theliquid crystal display device according to the embodiment.

In each of cases 1 and 2 as shown in FIG. 10, the frame frequency is1-10 Hz. In this case, as shown in FIG. 5, it is possible to reduce aflicker by applying a TFT whose off-leak current is 1×10⁻¹⁵ A or less,and also matching impedances of a liquid crystal and an alignment filmto each other.

FIG. 11 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 1 and 2 in the liquid crystaldisplay device according to the embodiment. In the regions indicatedwith respect to cases 1 and 2, the absolute value of a change rate ofluminance is 0.01 or less. In case 1, the resistivity of the liquidcrystal falls within the range of 5×10¹³ to 5×10¹⁴ Ω·cm, and that of thealignment film falls within the range of 5×10¹³ to 5×10¹⁴ Ω·cm. In case2, the resistivity of the liquid crystal falls within the range of1×10¹³ to 5×10¹³ Ω·cm, and that of the alignment film falls within therange of 1×10¹³ to 5×10¹³ Ω·cm.

In each of cases 3 and 4 as shown in FIG. 10, the frame frequency is10-20 Hz. In this case, as shown in FIG. 5, it is possible to reduce aflicker by applying a TFT whose off-leak current is 1×10⁻¹⁵ A or less,and matching impedances of a liquid crystal and an alignment film toeach other.

FIG. 12 is a view showing regions regarding impedances of the liquidcrystals and alignment films in cases 3 and 4 in the liquid crystaldisplay device according to the embodiment. In the regions indicatedwith respect to cases 3 and 4, the absolute value of a change rate ofluminance is 0.03 or less. In case 3, the resistivity of the liquidcrystal falls within the range of 5×10¹² to 5×10¹⁴ Ω·cm, and that of thealignment film falls within the range of 5×10¹² to 5×10¹⁴ Ω·cm. In case4, the resistivity of the liquid crystal falls within the range of1×10¹² to 5×10¹² Ω·cm, and that of the alignment film falls within therange of 1×10¹² to 5×10¹² Ω·cm.

In each of cases 5 and 6 as shown in FIG. 10, the frame frequency is10-20 Hz, and the TFT off-leak current is 1×10⁻¹⁵ to 1×10⁻¹⁴ A. In thiscase, as shown in FIG. 5, the luminance fluctuation value is great, anda flicker is thus visibly recognized. Thus, the impedances of the liquidcrystal and the alignment film are set so that the liquid crystalholding voltage is increased as shown in case 3 in FIG. 7B. Thereby, itis possible to reduce the flicker by compensating for decreasing of theluminance due to the TFT off-leak with increasing of the luminance dueto the impedance mismatch between the liquid crystal and the alignmentfilm.

FIG. 13 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 5 and 6 in the liquid crystaldisplay device according to the embodiment. In regions indicated withrespect to cases 5 and 6 (regions defined by solid lines), the regionsindicated with respect to cases 3 and 4 (regions defined by dottedlines) are shifted leftwards. That is, the luminance is increased bysetting the resistivity of the alignment film to a low value. In case 5,the resistivity of the liquid crystal falls within the range of 5×10¹²to 5×10¹⁴ Ω·cm, and that of the alignment film falls within the range of5×10¹¹ to 5×10¹³ Ω·cm. In case 6, the resistivity of the liquid crystalfalls within the range of 1×10¹² to 5×10¹² Ω·cm, and that of thealignment film falls within the range of 1×10¹¹ to 5×10¹¹ Ω·cm.

In each of cases 7 and 8 as shown in FIG. 10, the frame frequency is10-20 Hz, and the TFT off-leak current is 1×10⁻¹⁴ to 1×10⁻¹³ A. In thiscase, as shown in FIG. 5, the luminance fluctuation value is great, anda flicker is thus visibly recognized. Thus, the impedances of a liquidcrystal and an alignment film are set so that the liquid crystal holdingvoltage is increased as shown in case 3 in FIG. 7B. Thereby, it ispossible to reduce the flicker by compensating for decreasing of theluminance due to the TFT off-leak with increasing of the luminance dueto the impedance mismatch between the liquid crystal and the alignmentfilm.

FIG. 14 is a view showing regions regarding impedances of liquidcrystals and alignment films in cases 7 and 8 in the liquid crystaldisplay device according to the embodiment. In regions indicated withrespect to cases 7 and 8 (regions defined by solid lines), the regionsindicated with respect to cases 3 and 4 (regions defined by dottedlines) are further shifted leftwards. That is, the luminance is furtherincreased by setting the resistivity of the alignment film to a lowervalue. In case 7, the resistivity of the liquid crystal falls within therange of 5×10¹² to 5×10¹⁴ Ω·cm, and that of the alignment film fallswithin the range of 1×10¹¹ to 1×10¹³ Ω·cm. In case 8, the resistivity ofthe liquid crystal falls within the range of 1×10¹² to 5×10¹² Ω·cm, andthat of the alignment film falls within the range of 5×10¹⁰ to 1×10¹¹Ω·cm.

In case 9 as shown in FIG. 10, the frame frequency is 1-10 Hz. In thiscase, it is possible to reduce a flicker by applying a TFT whoseoff-leak current is 1×10⁻¹⁵ A or less, and matching the impedances of aliquid crystal and an alignment film to each other, as shown in FIG. 5.

FIG. 15 is a view showing regions regarding of impedances of a liquidcrystal and an alignment film in case 9 in the liquid crystal displaydevice according to the embodiment. In a region indicated with respectto case 9, the absolute value of a change rate of luminance is 0.01 orless. In case 9, the resistivity of the liquid crystal falls within therange of 5×10¹³ to 5×10¹⁴ Ω·cm, and that of the alignment film fallswithin the range of 1×10¹⁴ to 5×10¹⁴Ω·cm. It is possible to maintain asatisfactory flicker characteristic by setting the lower limit of theresistivity of the alignment film to a high value, as compared with case1, even if the resistivity of the alignment film varies with the passageof time.

Also, the inventors clarified that the resistance of the alignment filmlowers due to the entry of moisture from a panel peripheral portion. Theentry of moisture from the panel peripheral portion easily occurs in thecase where a peripheral seal material provided in an area other than aneffective display area is made thin. In the case where the width of theseal material was 1.5 mm or less, it was possible to set the absolutevalue of the change rate of luminance to 0.01 or less even under strictenvironment conditions such as a high-humidity environment, by settingthe resistivities of a liquid crystal and an alignment film to those ofcase 9.

In case 10 as shown in FIG. 10, the frame frequency is 10-20 Hz. In thiscase, it is possible to reduce a flicker by applying a TFT whose TFToff-leak current is 1×10⁻¹⁵ A or less, and matching the impedances of aliquid crystal and an alignment film to each other, as shown in FIG. 5.

FIG. 16 is a view showing regions regarding impedances of a liquidcrystal and an alignment film in case 10 in the liquid crystal displaydevice according to the embodiment. In a region indicated with respectto case 10, the absolute value of a luminance fluctuation rate is 0.03or less. In case 10, the resistivity of the liquid crystal falls withinthe range of 5×10¹² to 5×10¹⁴ Ω·cm, and that of the alignment film fallswithin the range of 1×10¹³ to 5×10¹⁴ Ω·cm. Case 10 is effective in thecase where the width of the seal material is 1.5 mm or less, as in case9. That is, it was possible to set the absolute value of the luminancefluctuation rate to 0.03 or less even under strict environmentconditions against the entry of moisture from the panel peripheralportion, such as a high-humidity environment.

As explained above, it is possible to achieve a liquid crystal displaydevice in which a flicker is restricted, by combining an appropriateframe frequency, appropriate TFT off-leak current and appropriateimpedances of a liquid crystal and an alignment film.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A lateral electric-field type of liquid crystaldisplay device comprising: a display panel in which a plurality ofliquid crystal pixels and TFTs are arranged in a matrix, the TFTs beingconfigured to perform or stop application of pixel voltages to theliquid crystal pixels; and a controller configured to performintermittent driving to rewrite image signals to the liquid crystalpixels, wherein a frame frequency falls within a range of 1 Hz to 10 Hz,an off-leak current of each of the TFTs has a value of 1×10⁻¹⁵ A orless, a resistivity of a liquid crystal applied to each of the liquidcrystal pixels and a resistivity of an alignment film applied to theliquid crystal pixels both fall within one of a first range and a secondrange, the first range being set as a range of 1×10¹³ to 5×10¹³ Ω·cm,the second range being set as a range of 5×10¹³ to 5×10¹⁴ Ω·cm, and arelationship of “R1×C1≈R2×C2” is satisfied, where R1 is a resistance ofthe liquid crystal with respect to said each liquid crystal pixel, C1 isa capacity of the liquid crystal with respect to said each liquidcrystal pixel, R2 is a resistance of the alignment film with respect tosaid each liquid crystal pixel, and C2 is a capacity of the alignmentfilm with respect to said each liquid crystal pixel.
 2. A lateralelectric-field type of liquid crystal display device comprising: adisplay panel in which a plurality of liquid crystal pixels and TFTs arearranged in a matrix, the TFTs being configured to perform or stopapplication of pixel voltages to the liquid crystal pixels; and acontroller configured to perform intermittent driving to rewrite imagesignals to the liquid crystal pixels, wherein a frame frequency fallswithin a range of 10 Hz to 20 Hz, an off-leak current of each of theTFTs has a value of 1×10⁻¹⁵ A or less, a resistivity of a liquid crystalapplied to the liquid crystal pixels and a resistivity of an alignmentfilm applied to the liquid crystal pixels both fall within one of afirst range and a second range, the first range being set as a range of1×10¹² to 5×10¹² Ω·cm, the second range being set as a range of 5×10¹²to 5×10¹⁴ Ω·cm, and a relationship of “R1×C1≈R2×C2” is satisfied, whereR1 is a resistance of the liquid crystal with respect to said eachliquid crystal pixel, C1 is a capacity of the liquid crystal withrespect to said each liquid crystal pixel, R2 is a resistance of thealignment film with respect to said each liquid crystal pixel, and C2 isa capacity of the alignment film with respect to said each liquidcrystal pixel.